1. Field of the Invention
The present invention relates to an improvement in a process for producing a superconducting thin film; more particularly, it relates to a process for producing a superconducting thin film at relatively lower temperature without heating a substrate on which the thin film is deposited.
The process according to the present invention is useful to produce a superconducting thin film having a high critical temperature and a uniform composition on such a substrate that is sensitive to heat.
2. Description of the related art
Superconductivity is a phenomenon which is explained as a kind of phase change of electrons under which the electric resistance becomes zero and perfect diamagnetism is observed.
In the field of electronics, a variety of superconducting devices are known. A typical application of a superconducting device is the Josephson device in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconductors. The tunnel junction type Josephson device is expected to be a high-speed and low-power consuming switching device owing to smaller energy gap of the superconducting material. It is also expected to utilize the Josephson device as a highly sensitive sensor or detector for sensing very weak magnetic fields, microwaves, radiant rays or the like, since variation of electromagnetic waves or magnetic fields is reflected in a variation of the Josephson effect and can be observed as a precise quantum phenomenon. Development of superconducting devices such as high-speed logic units or no powerloss wiring materials is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching the upper limit of cooling capacity with increase of the integration density in order to reduce energy consumption. However, the critical temperature "Tc" of superconductivity did not exceed 23.2 K of Nb.sub.3 Ge, which was the highest Tc for the past ten years.
The possibility of existence of a new type of superconducting material having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 180].
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La,Sr].sub.2 CuO.sub.4 which is called a K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type compound oxides show higher Tc's such as 30 K, which are extremely higher than known superconducting materials.
It was also reported in February 1987 that C. W. Chu et al. discovered, in the United States of America, another superconducting material of so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having a critical temperature of about 90 K. And hence, the possibility of existence of high-temperature superconductors has burst onto the scene.
The superconductor properties of the above-mentioned new type compound oxide superconductors are influenced by the oxygen deficiency in the crystal. In fact, if the oxygen deficiency does not exist in the crystal, high Tc cannot be observed and the difference between the on-set temperature and the temperature where perfect zero resistance is observed becomes large.
Thin films composed of the above-mentioned new type superconducting materials can be prepared by sputtering techniques, for example, RF magnetron sputtering or low pressure sputtering in which a sintered block composed of a superconducting compound oxide is used as a target. For example, a powdered material mixture comprising Y.sub.2 O.sub.3, BaCO.sub.3 and CuO is sintered to obtain a sintered block of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 which is used as a target.
The conventional sputtering technique for preparing a thin film composed of the ceramics type oxide supercondutors is disclosed in Japanese patent laid-open No. 56-109824. This patent teaches that the superconducting thin film which is represented by the formula BaPb.sub.1-x Bi.sub.x O.sub.3 (in which 0.05.ltoreq.x.ltoreq.0.35) is prepared by a high-frequency sputtering technique which is effected in an oxygen-containing atmosphere.
In the conventional sputtering technique, however, the substrate must be heated above 600.degree. C. during the sputtering stage. In fact, if the substrate is not heated above 600.degree. C., the thin film deposited on the substrate does not show desired superconducting properties because of poor crystallization of the compound oxide. In other words, heating of the substrate is indispensable in the conventional sputtering method. However, such a sputtering method is not applicable to a substrate that has a large difference in thermal expansion coefficient from that of the superconducting thin film. It is also impossible to use a heat-sensitive substrate whose properties are degraded at high temperatures. For example, it is difficult to use a single crystal of semiconductor material as a substrate because metal components in the compound oxide of which the superconducting thin film is composed diffuse into the single crystal, so that the properties of the thin film and the single crystal are changed. This means that the conventional process cannot be used in such combinations of superconducting compound oxide and semiconductor substrate which are very important for application of the superconducting compound oxides to the fabrication of superconducting transistors or the like.
The present applicants already proposed several processes for preparing the thin films of the high-Tc superconductor in the following patent applications: U.S. patent application Ser. No. 152,714 filed on Feb. 2, 1988, U.S. patent application Ser. No. 167,895 filed on Mar. 13, 1988, U.S. patent application Ser. No. 195,145 filed on May 12, 1988, now U.S. Pat. No. 5,900,716, U.S. patent application Ser. No. 195,147 filed on May 18, 1988, U.S. patent application Ser. No. 200,206 filed on May 31, 1988, now U.S. Pat. No. 4,996,185 or the like. Although the processes disclosed in these patent applications are themselves useful and satisfactory, it is still desirable to lower the substrate temperature during deposition of the thin film of superconductor.
Therefore, an object of the present invention is to provide a process which can be effected at a relatively lower temperature without heating the substrate and which is applicable for substrates composed of a single crystal of semiconductor material.